In the prior art, the use of silane treated clays as fillers for polymers or elastomerics is known. Typically, treated clays employing sulfur functional silanes are utilized in sulfur cured elastomeric systems requiring properties such as high tensile strength, high modulus or the like. Sulfur cured elastomers are often found in automotive applications such as tires, i.e., carcass, tire tread and white side walls, belts, hoses or the like.
Peroxide cured elastomeric systems are often used in jacketing applications, such as for wire and cable, and in specialty goods such as gaskets. Typically, these types of elastomeric systems seek good compression set properties. Vinyl functional silanes have been used in these applications.
With ever increasing competition in the elastomer industry, more and more applications are being developed which need high levels of reinforcement, either in terms of modulus, tensile strength, tear or compression set. To date, silica or carbon black fillers have been the only types of fillers which could provide the desired level of reinforcement. However, both of these filler systems are not without their disadvantages. Carbon black generally cannot be used in applications wherein the elastomer compound needs to be pigmented (i.e., white or non-black). In addition, a very fine particle size carbon black is needed to provide high levels of reinforcement and these carbon blacks can be extremely expensive. Further, in many tire related applications carbon blacks are known to contribute to higher heat build-up properties, as compared to clays, which can have deleterious effects on the service life of the tire.
Using a precipitated or fumed silica as a filler also contributes greatly to the cost of the compound since these silicas are often extremely expensive on a per pound basis. Moreover, they are difficult to process in elastomeric systems. Since silica fillers have extremely high surface areas, they are highly absorptive. When mixed with a given elastomeric compound, the silicas tend to absorb the oils, plasticizers or the like in the compound and make it difficult to mix the compound. This characteristic can often lead to poor filler dispersion thereby reducing expected physical properties. The use of high levels of precipitated silica in tire tread compounds provides excellent rolling resistance properties, but it is also known to cause the build-up of undesirable static charge such that they require the co-use of other semi-conductive fillers. Ideally, these replacement fillers should have virtually no deleterious effects on rolling resistance and rubber physical properties as compared to silica.
However, if one were seeking to produce a non-black elastomeric compound having a high level of reinforcement, silica and its attendant disadvantages would be the only choice. Carbon black cannot be used due to the need for pigmentation or color in the compound.
Silicas have been combined with various silanes for use in elastomer systems. U.S. Pat. No. 5,008,305 to Kennan et. al. describes a reinforcing silica for use in silicone elastomers. The reinforcing silica is prepared by treating the dry silica with a combination of both phenylalkoxysilane and vinylalkoxysilane. This combination of surface treatment improves compression set and heat aging in silicone elastomers. This art differs from the present invention in that Kennan et. al. use a silica as the reinforcing agent and that both phenyl and vinyl functional silanes are added to the silica in pure form rather than as emulsions. Further, both phenyl and vinyl functional silanes are required in the prior art composition as opposed to the use of a single functional silane for surface treatment in the present invention. Lastly, it is required that these silanes be volatile in order to observe the benefits. Volatility is unimportant in the present invention.
U.S. Pat. No. 4,714,733 describes a rubber composition containing an ethylene-propylene rubber, an organopolysiloxane having at least two alkenyl groups per molecule, a silica filler, an alkoxysilane, and a thiocarbamyl-containing organosilane. This prior art composition exhibits improved compression set and heat aging. This composition differs from the present invention in that the prior art requires the use of a thiocarbamyl-containing organosilane and the filler is a silica, not a surface treated kaolin clay.
Heretofore, silane treated clays have had limited utility in elastomeric applications requiring high performance because of their relatively low reinforcing benefits. Their ability to replace or extend high performance fillers, such as carbon black or silica, has been modest at best. Known silane treated clays for use in elastomer systems not requiring high performance include the Nucap.TM. and Nulok.TM. clays manufactured by J. M. Huber Corporation of Macon, Ga. The Nucap.TM. silane treated clays use a sulfur functional silane in treatment levels up to about 0.5% by weight of the silane based on dry clay. Exemplary of these sulfur functional silanes include a mercaptosilane, a thiocyanatosilane or a bridging tetrasulfane silane. The Nucap.TM. treated clays are therefore mainly targeted for use in sulfur-cured rubber systems. In comparison, the Nulok.TM. treated clays utilize various amino functional silanes in treatment levels up to about 1.0% by weight and these fillers are mainly used in peroxide-cured compounds. These Nucap.TM. and Nulok.TM. products, and their competitive counterparts, can be based on kaolin clay substrates ranging from fine particle size waterwashed clays, to waterwashed delaminated clays of relatively coarse particle size to various airfloat clays.
Up to the present, it was well recognized that increasing the amount of sulfur functional silanes on the clay did not necessarily increase the given performance of a given elastomeric system in a proportional manner. Diminishing incremental performance benefits are provided as silane treatment levels are increased. Thus, the silane treatments have been held to the levels noted above, e.g., about 0.5% by weight and below based on cost/performance considerations.
Besides the inability to provide a high level of performance in elastomeric systems, clay or current treated clays have also presented a problem in regards to their inherent higher specific gravity than that of silica or carbon black. The specific gravity of kaolin clay is 2.6 whereas the specific gravity of silica is about 2.0 to 2.2. Carbon black's specific gravity is about 1.8. In rubber compounds where density is critical, a treated clay cannot be substituted for carbon black or silica on a one to one weight basis while still meeting the density requirements. In other words, less clay must be used than a given phr amount of carbon black or silica to meet the density requirement. In addition, the reduced weight amount of clay must still be able to impart the same filler performance characteristics as the carbon black or silica. Conversely, if the filled rubber compounds are to be formulated to yield equal hardness then about 1.6 parts of clay or treated clay are normally required to replace every 1 part of carbon black while needing to still maintain other physical properties like modulus, tensile strength and tear. At a weight ratio of 1.6/1, this puts treated clays at a cost/performance disadvantage as extenders for larger particle size of soft carbon blacks unless the silane treated clays provide a very high level of performance.
In view of the disadvantages noted above with presently available silane treated clay products as well as the limitations of silica and carbon black as fillers in elastomeric systems, a need has developed to provide a silane treated clay product which can be used as a highly effective reinforcement for elastomeric systems.
The present invention solves this need by providing a method of making a silane treated clay and product therefrom which can be used as a reinforcing filler or extender in elastomeric systems to achieve high performance characteristics. The silane treated clays of this invention can be made by first emulsifying the functional silane prior to surface treating the clay.
Silanes have been used in dispersed or emulsified form in applications other than those employing clays.
Patent JP-06285363 describes the production of hydrophobic fine particles of an inorganic compound (more specifically particles of TiO.sub.2 pigment) by combining an aqueous dispersion of the inorganic compound with surfactant and alkylsilane for the purpose of obtaining a silicone polymer coating on the surface of fine powders. While the above patent describes a hydrophobic inorganic fine particle composition and a process to produce such a composition, the compositions of this present invention differ from the above by our demonstrated examples of unexpectedly high gains in cured elastomer reinforcing properties using significantly lower levels of silane treatments which are outside the scope of this prior art. In addition, the focus of this prior art was on the use of non-functionalized alkylsilanes as opposed to the functional silanes utilized in the present invention.
The technique of using an amino functional silane emulsion to treat an aqueous mineral slurry is described in U.S. Pat. No. 4,525,281. The treated mineral has improved dewatering properties. As with the current invention, a mineral is treated with a silane emulsion. However, the effective silanes of this present invention are not amino functional silanes, but rather vinyl and sulfur functional silanes which are required to chemically interact with both the kaolin clay and the elastomer. The unexpectedly high elastomer reinforcement benefits of the current invention could not have been predicted from the dewatering benefit described by the prior art.
A silane emulsion is described in U.S. Pat. No. 4,937,104 which is useful for making building material surfaces hydrophobic. The emulsion consists of alkyltrialkoxysilane in aqueous alcohol. Although this prior art and the current invention use silane emulsions for surface treatment, the current invention requires functional silanes to achieve the reinforcing properties in elastomers. Further, the observed hydrophobicity benefit in the prior art is unrelated to the reinforcing properties observed in the current invention.